/Engine/Source/Runtime/HeadMountedDisplay/Public/IHeadMountedDisplayModule.h
currently, for mac,GetGraphicsAdapterLuid()is used to return a device index (how the function "GetGraphicsAdapter" used to work), not a ID... eventually we want the HMD module to return the MTLDevice's registryID, but we cannot fully handle that until we drop support for 10.12 NOTE: this is why we use -1 as a sentinel value representing "no device" (instead of 0, which is used in the LUID case)
virtual uint64 GetGraphicsAdapterLuid()